Does Ceres exist in the underground ocean?

After arriving at the dwarf planet Ceres in 2015, NASA’s Dawn spacecraft detected some strange phenomena where there was ice flowing and erupting like magma. In the final phase of the mission (the exploration mission ended in 2018), the Dawn swept the area within 35 kilometers of the Occator Crater in its orbit. This is a 92 kilometers wide crater, which is scattered with bright deposits of sodium carbonate and other minerals.

Recently, 3 journals in the "Nature" series presented the data analysis results of a mission with 7 papers. These papers provided the most complete description of the history of Aucheres crater so far, and confirmed the previous relevant It is speculated that there may be liquid water under the surface of Ceres, but the extent of the area where the water exists is still unknown.

　　 "This is the last set of papers published by the project, and the detection data confirmed the team's conjecture." Carol Raymond said she is a researcher at NASA's Jet Propulsion Laboratory (JPL) and the associate principal researcher of Dawn. "Now these data are public and everyone can view it. I believe we will have more understanding in the future."

　　 In the early stages of the "Dawn" mission, researchers speculated that the minerals in the sediment were brought to the surface by liquid water, and then the liquid water evaporated. The last of these sediments is only 4 million years old, and the impact that caused the Aucheris Crater occurred about 20 million years ago. This time difference makes it impossible for scientists to determine whether liquid water initially exists in the form of ice and melts after being affected by the heat of the impact, or whether it always exists underground as liquid water.

　　 In order to find the answer, the research team recorded the speed changes of the spacecraft in its orbit to establish a detailed map of the gravity field of Ceres, thereby revealing the density of different crustal regions. Then, Raymond and colleagues combined the density distribution map of the crust with the heat transfer model of the ice-rich crust to reconstruct the impact results of the Orchris Crater. Raymond said this discovery "opens up a new way" that can be used to think about the geological structure of Ceres (the largest celestial body in the asteroid belt) and frozen celestial bodies. The energy from the impact formed a "melting chamber" of liquid water near the surface of Ceres and ruptured the crust. Raymond said the cracks connect the "melting chamber" to a liquid water reservoir 35 kilometers underground. Over the course of millions of years, as the melting chamber gradually freezes, the brine rises through the crack network. After reaching the ground, the water quickly vaporizes in a near-vacuum environment, leaving behind sodium carbonate and other salts.

　　High-resolution data allows researchers to study the composition of these bright deposits more closely. They discovered the existence of a mineral called hydrohalite, which is essentially a crystal formed from table salt and ice. This mineral is common in the sea ice of the earth, but has never been found elsewhere in the solar system. They calculated that once the ice salt is exposed on the surface of Ceres, the ice in it will disappear in about 100 years, leaving behind solid sodium chloride (the "Dawn" instrument cannot currently detect it). Therefore, the presence of ice salt indicates that there is still salt water that continues to flow to the surface of the planet. "We confirmed that this is the ongoing geological activity of Ceres," said Raymond, who is the co-author of these seven papers.

　　 Julie Castillo-Rogez is a planetary scientist at NASA’s Jet Propulsion Laboratory and co-author of 6 of these articles. In a review article in "Nature-Astronomy", she stated that Ceres is a "sea world". "Based on the results of early detection missions, we suspect that there is liquid on Ceres," she said, "but now we have more evidence." But other scientists said that based on current data, the dwarf planet is considered to be a The ocean world is a bit exaggerated.

“I’m still skeptical about the existence of an underground ocean on Ceres,” said Jim Zimbelman, a geologist at the Smithsonian Institution. “The model is very good, but I need to see some clear geophysical evidence to show that the ocean is coming soon. The past even still exists.” Mikhail Zolotov of Arizona State University was a consultant for one of the studies, but he was not directly involved in the work. He agreed with the high-resolution gravity data, but the conclusion that Ceres has saltwater reservoirs may be a subjective guess. In 2009, he proposed that the crust of Ceres is porous and there is almost no ice. He still believes that this explanation is more credible than the observations made by Dawn. "The interpretation of many data is not unique," he said. "We don't need ice to explain these detection results."

　　 Lindy Elkins-Tanton is the lead researcher of the asteroid 16 Psyche mission to be conducted by NASA, and did not participate in the research of Ceres. She said that the evidence that Ceres has saltwater reservoirs is "credible", but she doubts whether it is large enough to be considered an ocean. She feels that the most important new discovery is that “the heat of the geological activity occurring in a small celestial body may come from impacts,” she said. “In fact, there is no need to rely on the original heat of the celestial body.” Elkins-Tanton said, which shows Similar geological processes have occurred on other asteroids and moons, possibly triggering chemical reactions that are thought to be pre-life.

　　 After completing the final stage of the exploration of Ceres, scientists are now considering the next task. Castillo-Rogez submitted a report on the next mission to NASA as part of the Decadal Survey on Planetary Science and Astrobiology. The survey will determine NASA's research priorities in the next 2023-2032. She and other scientists hope that the spacecraft will land on Aucheres Crater to study the composition of salt water in more detail. This project will be competed by other celestial bodies in the asteroid belt for the first time. Zimbelman said: "The challenge lies in whether the scientific goal of this plan can truly capture the attention of NASA and the scientific community and make them willing to give up exploring other celestial bodies in the asteroid belt."

　　 Planetary scientist Joseph O’Rourke of Arizona State University believes that the possibility of a saltwater layer on Ceres makes it "an absolutely attractive place worthy of future exploration." Elkins-Tanton also believes that it will be valuable to get more information about "salt water". But in her opinion, discussing other small geological details of Ceres in the paper cannot help scientists understand the solar system more broadly. "If you are interested in the specific geology of a small planet like Ceres, you can ask a million questions and do a million things." She said, "But my personal interest in science is actually based on Interest in big scientific issues."

　　 Whether or not NASA launches another spacecraft to Ceres, planetary scientists agree that the "Dawn" mission is groundbreaking. As the first spacecraft to complete orbit and detect two celestial bodies outside the Earth (hermes outlet), the engineering achievements of the Dawn are revolutionary," Zimbelman Say. These all show how you can go to unknown places and get surprising observations. This is the true meaning of planetary science.